The invention concerns an apparatus for supporting blades of axial-flow machines, in particular turbojet propulsion units.
Axial-flow machines are machines through which a fluid, for example a gas such as air, flows axially. Such machines have at least one rotor which is equipped with blades and which either gives off energy to the fluid by way of the blades and thus acts as a compressor or pump, or at the blades onto which the fluid performs work, this then involving a turbine. Both compressors and turbines are generally of a multi-stage construction. Each stage of such multi-stage axial-flow machines is formed by a plurality of rotor blades which are generally arranged in an annular configuration projecting radially outwardly around a central shaft. Such an arrangement of rotor blades is referred to as a rotor blade assembly. A multi-stage compressor or a multi-stage turbine have a plurality of rotor blade assemblies. Provided both between the individual rotor blade assemblies of a compressor or a turbine and also upstream of the first rotor blade assembly and downstream of the last rotor blade assembly there are usually guide blade assemblies which are each also formed by a plurality of radially extending but fixedly arranged guide blades.
Turbojet propulsion units as are used for driving aircraft usually have both a multi-stage compressor and also a multi-stage turbine. The compressor sucks air into the propulsion unit and compresses it. In that situation air flows substantially in the axial direction through the compressor and passes in the compressed condition into one or more combustion chambers. There, the compressed air is mixed with fuel and the fuel is burnt. That results in a further increase in pressure in the combustion chamber. The gases flow out of the combustion chamber into a turbine in which they expand and give off their energy to the rotor blades of the rotor blade assemblies. The rotor blade assemblies of the turbine are connected to a turbine shaft and drive it. The turbine shaft transmits its energy to the shaft of the compressor and in addition possibly to a propeller, a fan or another consumer of mechanical energy. The turbine of a turbojet propulsion unit also has fluid flowing therethrough substantially axially and parallel to the turbine shaft.
Gaps are respectively provided between the radially free ends of the stationary guide blade assemblies of a compressor or a turbine and the rotating rotor on the one hand and between the radially free ends of the moving rotor blades and the stationary housing surrounding the respective rotor blade assembly on the other hand, so that the stationary guide blades do not touch moving parts and the rotating rotor blades do not touch stationary parts of the compressor or the turbine. The magnitude of those gaps is one of the decisive criteria in regard to the operating characteristics and the overall efficiency of the turbine or the compressor. Large gap sizes or different gap dimensions in relation to the individual blades of the guide blade assemblies or the rotor blade assemblies have an adverse effect. In a conventional arrangement with a central rotor, the inwardly facing free ends of the guide blade assemblies and the outwardly facing free ends of the rotor blades must be subjected to fine machining after assembly of the blades. As the blades are generally secured to the housing of the rotor in a way which permits a slight degree of mobility of the blades, they must be moved into a stable position for the fine machining procedure after assembly. The fine machining procedures involves the assembled blades being fixed and ground. In order to achieve a blade position which is appropriate to the function involved, the individual blades should be prestressed during the final grinding operation.
As the spaces between the respective propulsion unit stages are very slight and there is a requirement for a vibration-free grinding operation, it is known and conventional for the air guide blades of one-piece compressor housings of jet propulsion units to be cast by means of a setting wax, to be fixed thereby, and then to be machined. The amount of time involved in setting up the wax and removing the wax amounts to more than 30 hours for five stages of a housing. The amount of time involved is correspondingly greater when dealing with housings with more stages.
In addition DE 34 02 066 discloses an arrangement for fixing rotor blades, which is distinguished in that the blades of a rotor blade assembly can be prestressed by means of a pressure medium and the prestressing force produced by the pressure medium is associated with each blade of the rotor blade assembly. Equal distribution of the prestressing force produced by the pressure medium is achieved by the known equal distribution of the hydrostatic pressure in a fluid. The use of compressed air as the fluid is described. It has been found that the fluid has an undesirable tendency to transmit oscillations and vibrations which occur in the grinding operation on one blade to the other blades. The arrangement therefore does not satisfy the requirement for grinding the blade ends, in a manner which is as free from vibration as possible. Even worse is the fact than an uncontrollablexe2x80x94even slightxe2x80x94pressure drop results in flaws and defects in the grinding operation and has the consequence of rendering useless the rotor blade assembly, with corresponding high costs.
The object of the present invention is to provide an apparatus for supporting blades, in particular of a jet propulsion unit, which permits propulsion unit blades which are to be machined to be fixed in the most advantageous possible fashion, and which very substantially avoids the disadvantages of the state of the art.
In accordance with the invention that object is attained by an apparatus of the kind set forth in the opening part of this specification, which is distinguished by at least one pair of axially mutually oppositely disposed support rings which are supported adjustably relative to each other for assembly between guide or rotor blades of adjacent compressor or turbine stages.
One of those support rings bears in the assembled condition for example against the end edges of all blades of a guide blade assembly or rotor blade assembly of a turbine or a compressor, while the other support ring bears against the nose edges of all blades of a guide blade assembly or rotor blade assembly of the adjacent stage of the turbine or compressor. The rotor blade assembly or guide blade assembly disposed between the adjacent guide blade or rotor blade assemblies is in the meantime removed. The two support rings bear in the axial direction against each other with an adjustable force and in that way prestress the blades of the adjacent guide blade or rotor blade assemblies, with an adjustable force.
The support rings are preferably segmented in the peripheral direction. In that way the support rings can be dismantled and can be easily fitted in particular in one-piece compressor housings.
The support rings preferably bear adjustably against each other by means of a plurality of support elements which are distributed over the periphery of the support rings. For that purpose each support element preferably includes at least one respective clamping pin or bolt and two clamping portions whose mutual spacing is adjustable by means of the clamping bolt. The clamping portions each preferably have two flat clamping bevels which extend at an angle relative to each other and the support rings each preferably have laterally delimited co-operating bevels against which the clamping bevels bear. The clamping elements can then be arranged in such a way that the clamping portions move towards each other when the bolt connecting them is tightened and at the same time by way of their clamping bevels they spread the two mutually oppositely disposed supports rings by virtue of the clamping bevels of the clamping portions sliding along the corresponding co-operating bevels of the support rings and urging the support rings apart. The flat clamping bevels with also flat co-operating bevels which extend in parallel relationship therewith on the support rings ensure a large surface area for transmission of the pressure forces which are transmitted by the clamping portions to the support rings. In addition the clamping portions cannot turn. Finally, the parallelism of the clamping and co-operating bevels guarantees a uniform clamping action and tilting of the support ring segments is excluded.
Preferably at least some segments of a support ring are provided at the side thereof which is towards the blades with an elastic covering which prevents damage to the blades and possibly compensates for tolerances.
Preferably two mutually oppositely disposed segments of a pair of support rings are loosely connected together by connecting elements. The connecting elements are preferably round pins which are received by bores in the mutually oppositely disposed segments. The connecting elements prevent the oppositely disposed segments from falling apart and thus facilitate assembly of the support rings.
Particularly good working results are achieved with a method of machining the free ends of guide or rotor blades of axial-flow machines by means of the support apparatus according to the invention, wherein the blades together with a housing or rotor carrying same are fitted into a receiving apparatus and the support rings are so assembled between the guide or rotor blades of adjacent compressor or turbine stages and upstream of the first stage thereof and the last stage thereof in such a way that the support forces caused by the support arrangements are received and carried by the receiving apparatus. Preferably in that case the housing which is to be machined or the rotor which is to be machined is fitted into the receiving apparatus, with the center line disposed perpendicularly. In the last-mentioned alternative form of the method, the receiving apparatus and the support arrangement are preferably initially pre-assembled and after pre-assembly of all support rings has been implemented, firstly the lowermost support ring and then progressively upwardly all further support rings are set to a desired support force.
The method is particularly suitable more especially in its preferred alternative embodiments for ensuring rapid and precise support of all blades which are to be machined.
The invention will now be described in greater detail by means of an embodiment with reference to the accompanying drawings.